Wireless cellular telecommunication networks are known. In this regard, reference is made to FIG. 1 which schematically shows a cellular communications network 2. The area covered by the network 2 is divided into cells 4. Each cell is provided with a base station 6. The base station 6 is arranged to communicate with mobile stations or other user equipment 8 located in the cells.
Various standards have been proposed for communication within the network. For example, one known standard is the GSM standard (global system for mobile communications). This is primarily designed for speech. The GPRS standard (general packet radio service) has been proposed for data. The GPRS standard has generally been proposed in the context of the GSM standard. The GSM standard allows circuit switched communication whilst the GPRS standard allows packet switched communication.
When a mobile station or user equipment moves, it can move from one cell into another. When that occurs, the user equipment or mobile station will generally stop communicating with the base station of its old cell and start communicating with the base station of the new cell. This is referred to as handoff or handover.
In the context of the GPRS standard, cell reselection procedures are specified. Cell reselection is where the mobile station or user equipment changes the cell (and the base station) with which it is associated. Currently, the standard specifies three cell reselection procedures. With the first two procedures NC0 and NC1, the mobile station makes the decision as to which cell the mobile station is to be associated. In the procedure NC2, the network makes the decision as to the cell with which the mobile station is to be associated. As will be appreciated, if the cells are overlapping or if the mobile station is located in border regions, different decisions can be made in different circumstances as to the cell with which the mobile station is to be associated.
The cell reselection procedures specified in the current GPRS standard are based on the assumption that the L2 link (—this is the TBF—temporary block flow link which is set up to carry the data blocks) is set up in the new cell after the cell has changed and after the mobile station has acquired the necessary system information to operate in the new cell. In other words, the mobile station changes the cell (and hence base station) with which it is associated and only then is the mobile station able to acquire the necessary information to allow it to operate. This has the disadvantage that this procedure is relatively slow. It can take some time for the mobile station to obtain the necessary system information messages and after that set up the TBF link. This causes a gap in transmission during a cell change which can last several seconds. This can cause a noticeable degradation in the quality of service. GPRS is often used for non-real time services. However, even in the context of the non-real time services, the gap of several seconds can still make an adverse impact on the quality.
In one modification to the standard, the concept of a network assisted cell change procedure was introduced. In this proposal, there was the aim of shortening the gap in transmission by sending the target cell (ie the new cell) system information via the old cell before terminating the link with that old cell and leaving the cell. The target cell information refers to System Information (SI) and Packet System Information (PSI) messages that the mobile must acquire before accessing the cell. These messages are usually broadcast on broadcast channels. Because the broadcast channel capacity is not very high these messages are broadcasted relatively seldom which causes long delay if the mobile station acquires this information by listening to the broadcast channel. The modification provides a mechanism where these messages can be sent during the ongoing TBF (Temporary Block Flow) via the source cell. In other words the mobile station does not have to listen to broadcast messages but gets this information during the ongoing TBF in the source cell. With this mechanism, the mobile station can set up the TBF link in the target cell without having any delay caused by the mobile station listening to information broadcast by the base station of the new cell in order to get the required system information messages. However, this proposed method still has the problem that there is a gap in transmission during a cell change as it takes some time to set up the TBF and resolve the contention resolution. This gap may still be long enough to provide a noticeable quality degradation in the provided levels of services.
It has been appreciated by the inventors that when a mobile station changes cell, the target cell and the current cell may be controlled by the same packet control unit. Currently, it is proposed to always reset the radio link control protocol entity or TBF. This is disadvantageous.